The following websites belong to major organizations that have environmental protection as a significant part of their activity. Whilst (mostly) not directly involved in the parish, they are well worth checking out.
1) English Heritage
The only organization of this type with a property in the parish - Old Sarum. English Heritage concentrates on the built environment and properties, however, it has some significant areas of land that provides important habitat for some creatures.
2) Wiltshire Wildlife Trust.
WWT owns 34 reserves in Wiltshire, with several close to our parish, and the nearest is The Devenish Reserve, which is walking distance away, just the far side of Little Durnford. Very steep, lovely views, wild flowers, butterflies and lots more.
Notable for its campaigning work protecting and supporting birds, the RSPB owns 150 reserves throughout the UK, where the emphasis is (of course) on creating ideal habitats for birds. The nearest reserve to us is Garston Wood, near Sixpenny Handley in Dorset.
4) National Trust
As with English Heritage, the National Trust tends to concentrate on the built environment and properties, but has some very important land areas. The nearest site to us is Stonehenge, but I suggest the most interesting from an environmental point of view is the Jurassic Coast in Dorset.
5) Woodland Trust
Owners of over 1000 ancient woodlands throughout the UK, with almost all of them giving free access to the public. Unfortunately, there are very few in our area, and the nearest one is Tinney's Firs which is near Woodfalls on the Hampshire/Wiltshire border.
6) English Nature
The Government Agency in charge of protecting and promoting wildlife, geology and wild places. They have designated the Avon River Valley as a Local Nature Reserve (LNR) and the part just to the south of our parish (around the sports centre and the reed beds) has been designated a Site of Special Scientific Interest (SSSI).